This invention relates to a billet of a cast aluminum-containing ferritic stainless steel having a refined grain structure particularly advantageous for producing a thin foil by a metal peeling process. More particularly, this invention relates to a method for manufacturing a peeling billet from a casting of said stainless steel, which comprises forging and annealing the casting under conditions effective to refine the grain structure to a size conducive for the peeling operation.
U.S. Pat. No. 4,331,631, issued to Chapman et al in 1982, describes the manufacture of an automotive catalytic converter comprising a metal foil carrying a ceramic coating impregnated with noble metal catalyst. The foil is formed of an aluminum-containing ferritic stainless steel comprising, by weight, about 15 to 25 percent chromium, about 3 to 6 percent aluminum and the balance mainly iron. The alloy may contain a minor constituent, such as yttrium, to improve adherence of an integrally formed oxide layer. As described in the patent, the foil is produced by a metal peeling process. A cylindrical billet of the steel is rotated about the longitudinal axis, while a cutting tool is urged against the peripheral surface to shear a continuous thin chip that forms the foil. The patent is directed to growing oxide whiskers on the foil to improve adhesion of the applied coating.
In peeling cast stainless steel, there is a tendency of the metal to tear, producing discontinuities in the foil. This is attributed to the relatively large grain size, typically exhibiting cross-sectional dimensions on the order of 10 millimeters. Billets formed from powder metal exhibit a finer grain size on the order of 90 microns that is more conducive to peeling. However, powder of the stainless steel is expensive to manufacture and requires complex processing, such as hot isostatic pressing. Thus, it is desired to form a billet from the cast metal.
Manufacturers have attempted to produce a peeling ring from a casting by a process comprising forging the casting to consolidate the metal and shape the ring. Such forging is typically carried out at an elevated temperature to facilitate metal deformation and may include a radial force applied to the peripheral surface, as well as an axial force to press the metal to a desired length. This elevated temperature forging is inherently accompanied by recrystallization. However, heretofore this recrystallization has resulted in only limited grain refinement, producing a grain size on the order of 1 millimeter, significantly greater than the optimum size for peeling.
Therefore, it is an object of this invention to provide a fine-grain billet of the type used in a metal peeling process and derived from cast aluminum-containing ferritic stainless steel, which cast metal is forged and annealed under conditions effective to produce a refined grain structure having a size comparable to a powder metal billet and conducive to improved peelability.